Magnetorheological fluid compositions and prosthetic knees utilizing same

ABSTRACT

The present invention relates in one embodiment to magnetorheological fluids utilized in prosthetic joints in general and, in particular, to magnetorheological fluids utilized in controllable braking systems for prosthetic knee joints. Preferred magnetorheological fluids of the present invention comprises polarizable iron particles, a carrier fluid, and optionally an additive. Preferred additives include, but are not limited to functionalized carrier fluids as well as derivatized fluoropolymers. Preferred carrier fluids include, but are not limited, to perfluorinated polyethers.

RELATED APPLICATIONS DATA

[0001] This application claims priority under 35 U.S.C. 119(e) fromprovisional application Serial No. 60/467,722 filed May 2, 2003, theentirety of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates in one embodiment tomagnetorheological fluids utilized in prosthetic joints in general and,in particular, to magnetorheological fluids utilized in controllablebraking systems for prosthetic knee joints.

[0004] 2. Description of the Related Art

[0005] Three types of variable-torque brakes have been employed inprosthetic knees in the past: (i) dry friction brakes where one materialsurface rubs against another surface with variable force; (ii) viscoustorque brakes using hydraulic fluid squeezed through a variable sizedorifice or flow restriction plate; and (iii) magnetorheological (MR)brakes or dampers where MR fluid (containing small iron particlessuspended in the fluid) is squeezed through a fixed orifice or flowrestriction plate, with viscosity of the fluid being varied in responseto an applied magnetic field. Each of these technologies, asconventionally practiced in the field of prosthetics, can pose certaindisadvantages.

[0006] Though dry friction brakes can generally provide a substantialtorque range for their size, undesirably, they are often difficult tocontrol. After extended use, the frictional pads tend to wear, therebychanging the frictional characteristics of the brake and the torqueresponse for a given commanded torque. Disadvantageously, this can causeunreliable damping performance, and hence adversely affect the gait ofthe amputee and also cause discomfort to the amputee. Consequently, dryfriction brakes may need frequent servicing and/or replacement whichundesirably adds to the cost.

[0007] Under high loading conditions, viscous torque brakes aresusceptible to leakage of hydraulic fluid and possibly other damage dueto excessive pressure build-up. Disadvantageously, this can result in anirreversible state, since once the brake unit is overloaded it cannotreturn to normal. Therefore, such a viscous torque brake for aprosthetic joint is prone to catastrophic failure, and hence can beunreliable and detrimental to the safety of an amputee.

[0008] In certain MR brakes and dampers, the interaction of the MR fluidwith the device causes increased pressure, seal deterioration, or acombination of the two. Another possible cause of these adverse effectsis decomposition of the MR fluid. Once the seals fail or the MR fluiddecomposes, the prosthetic knee is no longer suitable for use.

SUMMARY OF THE INVENTION

[0009] In accordance with preferred embodiments, there is provided amagnetorheological fluid (MR fluid) comprising polarizable particles, acarrier fluid, and optionally an additive. In one embodiment, thepolarizable particles comprise iron particles ranging in size from about0.1 to about 100 microns, preferably from about 0.2 to about 50 microns,from about 0.4 to about 10 microns, or from about 0.5 to about 9microns, but also including about 0.3, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4,5, 6, 7, 8, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 40, 60, 70,80, and 90 microns, and ranges encompassing such sizes. In certainembodiments, iron particles comprise from about 1 to about 60% (v/v) ofthe total MR fluid volume, preferably from about 10 to about 50% (v/v),from about 20 to about 40% (v/v), but also including about 5, 15, 25,30, 35, 45, and 55% (v/v) and ranges encompassing such percentages.

[0010] Suitable candidates for carrier fluids include, but are notlimited to, silicone, hydrocarbon, esters, ethers, fluorinated esters,fluorinated ethers, mineral oil, unsaturated hydrocarbons, and waterbased fluids. In one embodiment, a preferred carrier fluid comprises analiphatic hydrocarbon. In another embodiment, a preferred carrier fluidcomprises a perfluorinated polyether (“PFPE”).

[0011] In one embodiment, a preferred additive comprises afunctionalized fluoropolymer, including, but not limited to, aparafluoropropene and oxygen polymerized amide derivative. In anotherembodiment, a preferred additive comprises a functionalized carrierfluid. Suitable candidates for monofunctionalized PFPE carrier fluidderivatives include, but are not limited to silane, phosphate, hydroxyl,carboxylic acid, alcohol and amine functions. Suitable candidates fordifunctional PFPE carrier fluid derivatives include, but are not limitedto, dihydroxyl, ethoxy ether, isocyanate, aromatic, ester and alcoholfunctions. In one embodiment, a preferred functionalized PFPE carrierfluid comprises a poly(hexafluoropropylene epoxide) with a carboxylicacid located on the terminal fluoromethylene group. In one embodiment,the additive comprises from about 0.1 to about 20% (v/v) of the carrierfluid, preferably from about 1 to about 15% (v/v), or from about 2 toabout 10% (v/v), but also including about 2.5, 3, 3.5, 4, 4.5, 5, 5.5,6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 11, 12, 13, 14, 16, 17, 18, and 19%(v/v) and ranges encompassing such percentages.

[0012] In one embodiment, a preferred MR fluid comprises about 28% (v/v)particles, and about 72% (v/v) fluid component wherein said fluidcomponent comprises about 5% (v/v) poly(hexafluoropropylene epoxide)with a carboxylic acid located on the terminal fluoromethylene groupadditive and about 95% (v/v) PFPE oil carrier fluid. In anotherembodiment, a preferred MR fluid comprises about 32% (v/v) particles,and about 68% (v/v) fluid component wherein said fluid componentcomprises about 5% (v/v) poly(hexafluoropropylene epoxide) with acarboxylic acid located on the terminal fluoromethylene group additiveand about 95% (v/v) PFPE oil carrier fluid. In another embodiment, apreferred MR fluid comprises about 28% (v/v) particles and about 72%(v/v) fluid component wherein said fluid component comprises about 5%(v/v) parafluoropropene and oxygen polymerized amide derivative additiveand about 95% (v/v) PFPE oil carrier fluid. In another embodiment, apreferred MR fluid comprises about 32% (v/v) particles and about 68%(v/v) fluid component wherein said fluid component comprises about 5%(v/v) parafluoropropene and oxygen polymerized amide derivative additiveand about 95% (v/v) PFPE oil carrier fluid. In embodiments containingPFPE oil, the PFPE oil may comprise substantially all one PFPE oil or amixture of one or more PFPE oils.

[0013] In one embodiment, an MR fluid is specifically designed for usein a shear mode device. For such a device, mechanically hard particlesare desired. The carrier fluid also desirably experiences a lessdramatic viscosity change over temperature changes as compared to otherfluids. This may be measured in terms of a viscosity index (test methodASTM D-2270) with preferred carrier fluids having higher viscosityindices. In one embodiment, preferred carrier fluids have viscosityindices preferably ranging from about 100 to about 340 based onkinematic viscosity at 104 and 212° F., from about 120 to about 320,from about 140 to about 300, but also including 160, 180, 200, 220, 240,255, 260, 280, and ranges encompassing these amounts. One embodimentthat accomplishes this includes a carrier fluid comprising one or morePFPE oils. For example, a preferred PFPE fluid, UNIFLOR™ 8510 has aviscosity index of 255. Without wishing to be bound by any theory, it isbelieved that preferred PFPE oils of certain embodiments demonstratedesirable viscosity indices due to their narrow distribution ofmolecular weights. Also, the MR fluid desirably does not produce asignificant amount of vapor in a sealed chamber so as to interfere withthe function of the device. In one embodiment, a fluid componentcomprising PFPE oil carrier fluid and a functionalized fluoropolymeradditive provides this property. Without wishing to be bound by anytheory, it is believed that preferred PFPE oils of certain embodimentsare less volatile, i.e. lower vapor pressures than other oils, becausethey have much higher molecular weights, e.g. about 2,000 to about15,000, and therefore do not produce a significant amount of vapor.

[0014] In addition, a shear mode device should provide sufficienttorque, for example torque production in one embodiment may be about 0.1to about 200 Newton-meters, more preferably about 0.3 to about 150Newton-meters, even more preferably about 0.5 to about 100Newton-meters, but also including about 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 20, 30, 40, 50, and 75 Newton-meters. In one embodiment, maintaininga sufficient ratio of particles, such as iron particles, to MR fluidprovides for this. In one embodiment, a suitable ratio is achieved whenthe iron particles comprise from about 1 to about 60% (v/v) of the totalMR fluid volume, preferably from about 10 to about 50% (v/v), morepreferably from about 20 to about 40% (v/v), but also including about 5,15, 25, 30, 35, 45, and 55% (v/v) and ranges encompassing thesepercentages.

[0015] In accordance with preferred embodiments, there is provided a MRfluid comprising polarizable particles, a carrier fluid, and optionallyan additive for use in a prosthetic knee, for example, a knee asdescribed in U.S. patent Publication 2001/0029400A1. In one embodiment,the prosthetic knee comprises at least two adjacent surfaces adapted forshear movement relative to one another wherein the MR fluid is containedbetween said adjacent surfaces. In one embodiment, the MR fluid used incombination with the knee comprises PFPE oil carrier fluid andparticles, such as polarizable particles described above. In oneembodiment, the polarizable particles comprise iron particles ranging insize from about 0.1 to about 100 microns, preferably from about 0.2 toabout 50 microns, more preferably from about 0.4 to about 10 microns,even more preferably from about 0.5 to about 9 microns, but alsoincluding about 0.3, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 11, 12,13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 40, 60, 70, 80, and 90 microns,and ranges encompassing these sizes. In certain embodiments, ironparticles comprise from about 1 to about 60% (v/v) of the total MR fluidvolume, preferably from about 10 to about 50% (v/v), more preferablyfrom about 20 to about 40% (v/v), but also including about 5, 15, 25,30, 35, 45, and 55% (v/v) and ranges encompassing such percentages.

[0016] In another embodiment, the MR fluid used in combination with aprosthetic knee optionally includes an additive. In one embodiment, apreferred additive comprises a functionalized fluoropolymer, morepreferably a parafluoropropene and oxygen polymerized amide derivative.In another embodiment, a preferred additive comprises a functionalizedcarrier fluid. Suitable candidates for monofunctionalized PFPE carrierfluid derivatives include, but are not limited to silane, phosphate,hydroxyl, carboxylic acid, alcohol and amine functions. Suitablecandidates for difunctional PFPE carrier fluid derivatives include, butare not limited to, dihydroxyl, ethoxy ether, isocyanate, aromatic,ester and alcohol functions. In one embodiment, a preferredfunctionalized PFPE oil comprises a poly(hexafluoropropylene epoxide)with a carboxylic acid located on the terminal fluoromethylene group. Inone embodiment, the additive comprises from about 0.1 to about 20% (v/v)of the carrier fluid, preferably from about 1 to about 15% (v/v) , morepreferably from about 2 to about 10% (v/v), but also including about2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 11, 12, 13,14, 16, 17, 18, and 19% (v/v), and ranges encompassing these amounts.

[0017] In another embodiment, the passage or cavity of the knee thatholds the MR fluid contains a volume of about 1 to about 10 ml,preferably from about 2 to about 9 ml, more preferably from about 3 toabout 8 ml, but also including about 4, 5, 6, and 7 ml, and rangesencompassing these volumes. In one embodiment, the MR fluid fills thecavity to about 70% of its capacity, but ranges from about 50 to about100% as well about 55, 60, 65, 75, 80, 85, 90 and 90% and rangesencompassing these amounts are also acceptable.

[0018] In another embodiment, the MR fluid used in combination with aprosthetic knee in shear mode in one embodiment utilizes a MR fluid thatis operable over a temperature range from about 10 to about 115° F., butalso including about 20, 30, 40, 50, 60, 70, 80, 90, 100, and 110° F.Operability in one embodiment depends on viscosity, wherein the carrierfluid desirably has a viscosity at 104° F. of about 10 to about 100 cSt(centistokes), more preferably about 30 to about 80 cSt, even morepreferably about 50 to about 70 cSt, but also including about 10, 20,25, 35, 40, 45, 55, 60, 65, 75, 85, 90, and 95 cSt.

[0019] Desirably, operation of a prosthetic knee in shear mode in oneembodiment preferably utilizes a carrier fluid with a pour pointpreferably ranging from about −70° C. to about −40° C., from about −65°C. to about −45° C., but also including about −50° C., −55° C., and −60°C., and ranges encompassing these temperatures. In another embodiment,operation of a prosthetic knee in shear mode preferably utilizes acarrier fluid with a percent volatility at 121° C. preferably rangingfrom about 0.01% to about 20%, from about 0.02% to about 15%, from 0.03%to about 12%, but also including about 0.05%, 0.08%, 0.1%, 0.2%, 0.3%,0.4%, 0.5%, 0.7%, 0.9%, 1%, 3%, 5%, 7%, 9%, 17%, and ranges encompassingthese percentages.

[0020] In one embodiment, a preferred MR fluid used in combination witha prosthetic knee in shear mode comprises about 28% (v/v) particles, andabout 72% (v/v) fluid component wherein said fluid component comprisesabout 5% (v/v) poly(hexafluoropropylene epoxide) with a carboxylic acidlocated on the terminal fluoromethylene group additive and about 95%(v/v) PFPE oil carrier fluid. In another embodiment, a preferred MRfluid used in combination with a prosthetic knee in shear mode comprisesabout 32% (v/v) particles, and about 68% (v/v) fluid component whereinsaid fluid component. comprises about 5% (v/v) poly(hexafluoropropyleneepoxide) with a carboxylic acid located on the terminal fluoromethylenegroup additive and about 95% (v/v) PFPE oil carrier fluid. In anotherembodiment, a preferred MR fluid used in combination with a prostheticknee in shear mode comprises about 28% (v/v) particles, and about 72%(v/v) fluid component wherein said fluid component comprises about 5%(v/v) parafluoropropene and oxygen polymerized amide derivative additiveand about 95% (v/v) PFPE oil carrier fluid. In another embodiment, apreferred MR fluid comprises about 32% (v/v) particles and about 68%(v/v) fluid component wherein said fluid component comprises about 5%(v/v) parafluoropropene and oxygen polymerized amide derivative additiveand about 95% (v/v) PFPE oil carrier fluid. In embodiments containingPFPE oil, the PFPE oil may comprise substantially all one PFPE oil or amixture of one or more PFPE oils.

[0021] All of these embodiments are intended to be within the scope ofthe invention herein disclosed. These and other embodiments of thepresent invention will become readily apparent to those skilled in theart from the following detailed description of the preferred embodimentshaving reference to the attached figures, the invention not beinglimited to any particular. preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIGS. 1 and 2 depict one embodiment of a prosthetic knee suitablefor use in preferred embodiments. FIGS. 1 and 2 correspond to FIGS. 4and 5, respectively, of U.S. patent Publication 2001/0029400A1(application Ser. No. 09/767,367), filed Jan. 22, 2001, entitled“ELECTRONICALLY CONTROLLED PROSTHETIC KNEE,” the entire disclosure ofwhich is hereby incorporated by reference herein. More specifically, thedescription of the drawings and the item numbers depicted in thedrawings are described in detail in the above referenced patentpublication.

[0023]FIGS. 3-5 illustrate dynamic viscosity curves for various carrieroils and MR fluid samples.

[0024]FIG. 6 illustrates a comparison of the viscosities of variouscarrier oils and MR fluids.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Disclosed herein are magnetorheological fluids (MR fluids)suitable for use in magnetorheological knee brakes or actuators. Moreparticularly, the disclosed MR fluids may be applicable to prostheticknee joints which operate in shear mode, for example, where the MR fluidis provided between adjacent surfaces, such as between parallel platesor in the annular space between inner and outer cylinders. Certainembodiments of a magnetorheological knee brake or actuator that mayemploy the MR fluids as described herein are described in U.S. patentPublication 2001/0029400A1 (application Ser. No. 09/767,367), filed Jan.22, 2001, entitled “ELECTRONICALLY CONTROLLED PROSTHETIC KNEE,” theentire disclosure of which is hereby incorporated by reference herein.FIGS. 1 and 2, corresponding to FIGS. 4 and 5 of U.S. patent Publication2001/0029400A1, also depict one embodiment of a magnetorheological kneebrake or actuator that may employ the MR fluids as described herein.Certain embodiments of a control scheme and system formagnetorheological knee brakes or actuators are described in copendingU.S. patent Publication 2002/0052663A1 (application Ser. No.09/823,931), filed Mar. 29, 2001, entitled “SPEED-ADAPTIVE ANDPATIENT-ADAPTIVE PROSTHETIC KNEE,” the entire disclosure of which ishereby incorporated by reference herein. It will be appreciated,however, that the MR fluids as described herein may have applicabilityto other devices which utilize MR fluids, including but not limited to,other devices operating in a shear mode.

[0026] In one embodiment, the magnetorheological fluid preferablycomprises a plurality of iron, ferrous or magnetic particles suspendedin fluid. These suspended particles form torque producing chains inresponse to an applied magnetic field. Thus, the magnetorheological (MR)fluid undergoes a rheology or viscosity change or variation, which isdependent on the magnitude of the applied magnetic field. In turn, thisvariation in the bulk fluid viscosity determines the magnitude of theshearing force/stress or torque generated, and hence the level ofdamping or braking provided by the prosthetic knee or other device.Typically, the bulk viscosity of the MR fluid increases with increasingstrength of the applied field. By controlling the magnitude of thismagnetic field, the rotary motion of an artificial limb is rapidly andprecisely adjusted and/or controlled, for example, to control theflexion and extension during swing and stance phases to provide a morenatural and safe ambulation for the amputee. Preferably the MR fluid hasone or more of the following properties: a high magnetic flux capacityand low magnetic remanence and low viscosity while having a largemagnetic field induced shearing stress so that, advantageously, aprosthetic knee in one embodiment, provides a wide dynamic torque range.

[0027] In one embodiment, the MR fluid preferably comprises a carrierfluid with polarizable ferrous or iron particles. As used herein, theterm carrier fluid is a broad term used in its ordinary sense andincludes embodiments wherein the specific carrier fluids described beloware the primary component and embodiments wherein the carrier fluidcomprises these specific fluids as well as additives described below. Inaddition, embodiments wherein the additives described below are theprimary carrier fluid are also contemplated. In one embodiment, such aswhen used between rotor-stator surfaces of U.S. 2001/0029400A1, theseparticles have a size on the order of a micron or a few microns. Ideallythe MR fluid exhibits shear rate thinning behavior where MR fluidviscosity decreases with increasing shear rate. This advantageouslyminimizes the viscous torque due to shearing of the MR fluid betweeneach rotor-stator pair under zero-field conditions (that is, when theelectromagnet is not energized), and hence allows for a larger operatingtorque range. Further, in one embodiment MR fluids used in combinationwith a prosthetic knee desirably exhibit low off-state viscosity andtherefore low off-state torque as torque is proportional to MR fluidviscosity. The viscosity of preferred MR fluids in certain embodimentsmay be altered by one or more of the following: increasing or decreasingthe particle loading, including an additive, changing the carrier fluid,or mixing two or more carrier fluids.

[0028] Suitable candidates for carrier fluids include, but are notlimited to, silicone, hydrocarbon, esters, ethers, fluorinated esters,fluorinated ethers, mineral oil unsaturated hydrocarbons, and waterbased fluids. In one embodiment, the carrier fluid comprisessubstantially all one fluid. In another embodiment, the carrier fluid isa mixture of one or more carrier fluids. In one embodiment, the carrierfluid preferably comprises an aliphatic hydrocarbon. In anotherembodiment the carrier fluid preferably comprises a perfluorinatedpolyether (PFPE), also known as perfluoropolyether, perfluoroalkyletheror perfluoropolyalkylether, fluid. In certain embodiments, a preferredPFPE oil comprises fluorine end capped branched homopolymers ofhexafluoropropylene epoxide with the following chemical structure:

[0029] Where n=10-60

[0030] In another embodiment, a preferred PFPE oil comprises a branchedPFPE containing pendent trifluoromethyl groups, (—CF₃), with thefollowing structure:

CF₃CF₂CF₂O—[CF(CF₃)CF₂—O—]_(n)CF₂CF₃

[0031] Where n=5-65

[0032] In another embodiment, a preferred PFPE oil comprises a linearPFPE with the following structure:

CF₃O—[CF₂CF₂—O—]_(z)[CF₂—O—]_(p)CF₃

[0033] Where the ratio of z:p is between about 0.5:1 and 2:1, and z+p isbetween about 40 and about 180. In another embodiment, a preferred PFPEoil comprises a linear PFPE with the following structure:

CF₃CF₂CF₂O—[CF₂CF₂CF₂—O—]_(n)CF₂CF₃

[0034] Where n=10-50

[0035] In another embodiment, a preferred PFPE oil comprisesperfluoropropylpolyether. As presently contemplated preferredperfluorinated polyethers may be purchased from Nye Lubricants(Fairhaven, Mass., USA) and include, but are not limited to, UNIFLOR™8510, UNIFLOR™ 8130, UNIFLOR™ 8140, UNIFLOR™ 8730 and UNIFLOR™ 8970.Suitable perfluorinated polyethers may also be purchased from E.I. duPont de Nemours and Company, (Wilmington, Del., USA) and include, butare not limited to, Krytox® GPL-103, Krytox® L-65 oil, Krytox® XP 1A4oil, Krytox® L-100, Krytox® 1525, Krytox® 1525S, and Krytox® 1531.

[0036] Other ingredients can be optionally added to the carrier fluidsof preferred embodiments to enhance the performance properties ofpreferred carrier fluids. In some embodiments, preferred additivesinclude, but are not limited to, functionalized carrier fluids. Inembodiments comprising perfluorinated polyethers, desirable additivescan also include, but are not limited to, functionalized PFPE oils aswell as derivatized fluoropolymers. Suitable candidates formonofunctionalized PFPE derivatives include, but are not limited tosilane, phosphate, hydroxyl, carboxylic acid, alcohol and aminefunctions. Suitable candidates for difunctional PFPE derivativesinclude, but are not limited to, dihydioxyl, ethoxy ether, isocyanate,aromatic, ester and alcohol functions. More specifically, in oneembodiment comprising perfluorinated polyethers, a preferredfunctionalized PFPE oil comprises a poly(hexafluoropropylene epoxide)with a carboxylic acid located on the terminal fluoromethylene group. Aspresently contemplated preferred functionalized PFPE oils are Krytox(157 FSL and Krytox® 157 FSM available from E.I. du Pont de Nemours andCompany, (Wilmington, Del., USA). In another embodiment, a preferredfluoropolymer comprises a parafluoropropene and oxygen polymerized amidederivative. As presently contemplated a preferred parafluoropropene andoxygen polymerized amide derivative additive is FOMBLIN DA 306 availablefrom Solvay Solexis (Thorofare, N.J., USA).

[0037] Suitable candidates for polarizable ferrous or iron particlesinclude, but are not limited to, particles ranging in size from about0.1 to about 100 microns, preferably from about 0.2 to about 50 microns,more preferably from about 0.4 to about 10 microns, even more preferablyfrom about 0.5 to about 9 microns, but also including about 0.3, 0.6,0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 25, 30, 40, 60, 70, 80, and 90 microns, and ranges encompassingthese sizes. In certain embodiments, preferred particles aremechanically hard. As presently contemplated preferred iron particlesare available from BASF AG (Ludwigshafen, Germany) and include, but arenot limited to, BASF Carbonyl Iron Powder OM, BASF Carbonyl Iron PowderHQ, BASF Carbonyl Iron Powder HS, BASF Carbonyl Iron Powder EW, BASFCarbonyl Iron Powder HS-I, and BASF Carbonyl Iron Powder HL-1. Othersuitable iron particles may also be purchased from ISP Corporation(Wayne, N.J., USA). Other suitable ferrous or iron particles well knownto those of skill in the art may also be used. In related embodiments,particles comprising magnetic or ferromagnetic materials other than ironmay be used alone or in combination with iron-based particles.

[0038] In accordance with a preferred embodiment, the MR fluidcomposition comprises polarizable iron particles, PFPE carrier fluid,and an additive. In one embodiment, the iron particles comprise fromabout 1 to about 60% (v/v) of the total MR fluid volume, preferably fromabout 10 to about 50% (v/v), more preferably from about 20 to about 40%(v/v), but also including about 5, 15, 25, 30, 35, 45, and 55% (v/v) andranges encompassing such percentages. To determine the weight ofparticles required to achieve the proper % (v/v), the required volume ismultiplied by the density of the particles. In one embodiment, theadditive comprises from about 0.1 to about 20% (v/v) of the carrierfluid, preferably from about 1 to about 15% (v/v), more preferably fromabout 2 to about 10% (v/v), but also including about 2.5, 3, 3.5, 4,4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 11, 12, 13, 14, 16, 17, 18,and 19% (v/v) and ranges encompassing such percentages. For example, inone embodiment a preferred MR fluid comprises about 28% (v/v) ironparticles and about 72% (v/v) fluid component wherein said fluidcomponent comprises about 5% (v/v) a parafluoropropene and oxygenpolymerized amide derivative additive and about 95% (v/v) PFPE carrierfluid. In another embodiment, a preferred MR fluid comprises about 32%(v/v) particles and about 68% (v/v) fluid component wherein said fluidcomponent comprises about 5% (v/v) parafluoropropene and oxygenpolymerized amide derivative additive and about 95% (v/v) PFPE oilcarrier fluid. In another embodiment, a preferred MR fluid comprisesabout 28% (v/v) particles, and about 72% (v/v) fluid component whereinsaid fluid component comprises about 5% (v/v) poly(hexafluoropropyleneepoxide) with a carboxylic acid located on the terminal fluoromethylenegroup additive and about 95% (v/v) PFPE oil carrier fluid. In anotherembodiment, a preferred MR fluid comprises about 32% (v/v) particles,and about 68% (v/v) fluid component wherein said fluid componentcomprises about 5% (v/v) poly(hexafluoropropylene epoxide) with acarboxylic acid located on the terminal fluoromethylene group additiveand about 95% (v/v) PFPE oil carrier fluid. In embodiments containingPFPE oil, the PFPE oil may comprise substantially all one PFPE oil or amixture of one or more PFPE oils.

[0039] The MR fluid ingredients may be combined in any order and mixedby any suitable means including, but not limited to, stirring,agitation, sonification or blending. In accordance with a preferredembodiment, additives are first mixed with carrier fluids and stirred.Carrier fluid is added to the iron particles and the ingredients arestirred. The particles are then dispersed using sonification. Theresulting MR fluid is then heated. A detailed example is provided belowin the example section.

[0040] When the MR fluids as described herein are used in combinationwith a prosthetic knee, for example, a knee as described in U.S. patentPublication No. 2001/0029400A1, certain characteristics of the fluid aswell as the knee may be desired. In one embodiment, such as shown inFIGS. 4 and 5 of U.S. 2001/0029400A1, a knee may contain a cavity orpassage for holding MR fluid between a plurality of rotors and stators.The number of rotors and stators in certain embodiments may be increasedor reduced in order to alter the off-state or low-end torque propertiesof the MR fluid used in combination with the knee. In one embodiment,the number of rotors and stators preferably range from about 50 to about90, preferably from about 55 to about 70, but also including about 57,59, 61, 63, 65, 67, and ranges encompassing these amounts. The kneecavity may contain a volume of about 1 to about 10 ml, preferably fromabout 2 to about 9 ml, more preferably from about 3 to about 8 ml, butalso including about 4, 5, 6, and 7 ml. In one embodiment, the MR fluidfills the cavity to about 70% of its total volume, but may range fromabout 50 to about 100% as well about 55, 60, 65, 75, 80, 85, 90 and 90%.The MR fluid advantageously demonstrates one or more of the following:relatively low volatility, stable viscosity, thermal stability, and astable composition. In addition, in certain embodiments it is desirablethat the cavity or passage containing the MR fluid does not exhibitundesirable pressure levels. Without wishing to be bound by any theory,it is believed that an unsuitable fluid may release gases or volatilizecausing pressure within the prosthetic knee to increase to anundesirable level. If the pressure is too high, the integrity of theprosthetic knee seals can be compromised. In certain embodiments it isdesirable that a prosthetic knee utilizing a MR fluid produces torque ofabout 0.1 to about 200 Newton-meters, more preferably about.0.3 to about150 Newton-meters, even more preferably about 0.5 to about 100Newton-meters, but also including about 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 20, 30, 40, 50, and 75 Newton-meters.

[0041] Desirably, operation of a prosthetic knee in shear mode in oneembodiment utilizes a MR fluid that is operable over a temperature rangefrom about 10 to about 115° F., but also including about 20, 30, 40, 50,60, 70, 80, 90, 100, and 110° F. Operability in one embodiment dependson viscosity, wherein the carrier fluid desirably has a viscosity at104° F/ (40° C.) of about 10 to about 100 cSt, more preferably viscosityof about 30 to about 80 cSt, even more preferably viscosity of about 50to about 70 cSt, but also including about 10, 20, 25, 35, 40, 45, 55,60, 65, 75, 85, 90, and 95 cSt. The viscosity of preferred MR fluids incertain embodiments may be altered by one or more of the following:increasing or decreasing the particle loading, including an additive,changing the carrier oil, or mixing two or more carrier oils.

[0042] In one embodiment, an MR fluid is specifically designed for usein a shear mode device. For such a device, mechanically hard particlesare desired. The carrier fluid also desirably experiences a lessdramatic viscosity change over temperature changes as compared to otherfluids. This may be measured in terms of a viscosity indices (testmethod ASTM D-2270) with preferred carrier fluids having higherviscosity indices. In one embodiment, preferred carrier fluids haveviscosity indices preferably ranging from about 100 to about 340 basedon kinematic viscosity at 104 and 212° F., from about 120 to about 320,from about 140 to about 300, but also including 160, 180, 200, 220, 240,255, 260, 280, and ranges encompassing these amounts. One embodimentthat accomplishes this includes a carrier fluid comprising one or morePFPE oils. For example, a preferred PFPE fluid, UNIFLOR™ 8510 has aviscosity index of 255. Without wishing to be bound by any theory, it isbelieved that preferred PFPE oils of certain embodiments demonstratedesirable viscosity indices due to their narrow distribution ofmolecular weights. Also, the MR fluid desirably does not produce asignificant amount of vapor in a sealed chamber so as to interfere withthe function of the device. In one embodiment, a fluid componentcomprising PFPE oil carrier fluid and a fluoropolymer additive providesthis property. Without wishing to be bound by any theory, it is believedthat preferred PFPE oils of certain embodiments are less volatile, i.e.lower vapor pressures than other oils, because they have much highermolecular weights, e.g. about 2,000 to about 15,000, and therefore donot produce a significant amount of vapor.

[0043] Desirably, operation of a prosthetic knee in shear mode in oneembodiment preferably utilizes a carrier fluid with a pour point (testmethod ASTM D-97) preferably ranging from about −70° C. to about −40°C., from about −65° C. to about −45° C., but also including about −50°C., −55° C., −60° C., and ranges encompassing these temperatures. ASTMD-97 method defines “pour point” as the lowest temperature at whichmovement of an oil is observed. In another embodiment, operation of aprosthetic knee in shear mode preferably utilizes a carrier fluid with apercent volatility at 121° C. (test method ASTM D-972) preferablyranging from about 0.01% to about 20%, from about 0.02% to about 15%,from about 0.03% to about 12%, but also including about 0.05%, 0.08%,0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.7%, 0.9%, 1%, 3%, 5%, 7%, 9%, 17%, andranges encompassing these percentages.

[0044] More specifically, when used in combination with a prostheticknee as previously disclosed, desirable MR fluids for use in certainembodiments comprise a carrier fluid and polarizable particles. Morespecifically, in one embodiment used in combination with a prostheticknee, the MR fluid comprises one or more PFPE oil carrier fluids andpolarizable particles. In one embodiment, the iron particles comprisefrom about 1 to about 60% (v/v) of the total MR fluid volume, preferablyfrom about 10 to about 50% (v/v), more preferably from about 20 to about40% (v/v), but also including about 5, 15, 25, 30, 35, 45, and 55%(v/v), and ranges encompassing these percentages.

[0045] In another embodiment, the MR fluid used in combination with aprosthetic-knee may optionally comprise an additive. Suitable additivesinclude, but are not limited to, flinctionalized carrier fluids as wellas fluoropolymers. In one embodiment, the iron particles comprise fromabout 1 to about 60% (v/v) of the total MR fluid volume, preferably fromabout 10 to about 50% (v/v), more preferably from about 20 to about 40%(v/v), but also including about 5, 15, 25, 30, 35, 45, and 55% (v/v),and ranges encompassing these amounts. In one embodiment, the additivecomprises from about 0.1 to about 20% (v/v) of the carrier fluid,preferably from about 1 to about 15% (v/v) , more preferably from about2 to about 10% (v/v), but also including 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6,6.5, 7, 7.5, 8, 8.5, 9, 9.5, 11, 12, 13, 14, 16, 17, 18, and 19% (v/v).For example, in one embodiment a preferred MR fluid used in combinationwith a prosthetic knee in shear mode comprises about 32% (v/v) particlesand about 68% (v/v) fluid component wherein said fluid componentcomprises about 5% (v/v) a parafluoropropene and oxygen polymerizedamide derivative additive and about 95% (v/v) perfluorinated polyethercarrier fluid. In another embodiment a preferred MR fluid used incombination with a prosthetic knee in shear mode comprises about 28%(v/v) particles and about 72% (v/v) fluid component wherein said fluidcomponent comprises about 5% (v/v) a parafluoropropene and oxygenpolymerized amide derivative additive and about 95% (v/v) perfluorinatedpolyether carrier fluid. In another embodiment, a preferred MR fluidused in combination with a prosthetic knee in shear mode comprises about32% (v/v) particles, and about 68% (v/v) fluid component wherein saidfluid component comprises about 5% (v/v) poly(hexafluoropropyleneepoxide) with a carboxylic acid located on the terminal fluoromethylenegroup additive and about 95% (v/v) PFPE oil carrier fluid. In anotherembodiment, a preferred MR fluid used in combination with a prostheticknee in shear mode comprises about 28% (v/v) particles, and about 72%(v/v) fluid component wherein said fluid component comprises about 5%(v/v) poly(hexafluoropropylene epoxide) with a carboxylic acid locatedon the terminal fluoromethylene group additive and about 95% (v/v) PFPEoil carrier fluid. In embodiments containing PFPE oil, the PFPE oil maycomprise substantially all one PFPE oil or a mixture of one or more PFPEoils.

[0046] The ingredients may be combined in any order and mixed by anysuitable means including, but not limited to, stirring, agitation,blending or sonification. In accordance with a preferred embodiment, theMR fluid is prepared as described above. Prior to loading into theprosthetic knee, the MR fluid is stirred under vacuum using a high speedstirrer to remove any dissolved gases. In a preferred embodiment, the MRfluid is heated to about 90 to about 160° F., more preferably to about100 to about 150° F., even more preferably to about 110 to about 130°F., but also including about 95, 105, 115, 120, 122, 125, 135, 140, 145,and 155° F., under vacuum prior to loading into the prosthetic knee.

[0047] In another preferred embodiment, the MR fluid is heated atambient pressure prior to being placed under vacuum. While under vacuum,agitation or stirring of the MR fluid is preferred but not required.After the MR fluid is released from the vacuum, the MR fluid is loadedinto the prosthetic knee. The loading of the prosthetic knee involvesadding the MR fluid to the knee and then placing the knee under vacuum.While under vacuum the knee is optionally agitated. To reduce the vacuumpressure, an inert gas is added into the vacuum chamber. Once, thevacuum is fully released, the prosthetic knee is removed and closed. Thevacuum fill process should be carefully monitored as exiting air mayblow enough MR fluid out of the funnel to require fluid volumereplenishment.

EXAMPLES Example 1 MR Fluid Preparation

[0048] To prepare the MR fluid, the additives were mixed with thecarrier fluids and stirred. Carrier fluid was added to the ironparticles and the ingredients were stirred. A Branson Digital Sonifier,Model 450, was used to disperse the iron particles in the carrier fluid.The MR fluid was then placed on the sonifier table, with the probeadjusted so that the majority of the probe was immersed in the MR fluidwithout touching the bottom of the mixture jar. The MR fluid was thensonicated for 1.5 minutes at 50% intensity while the sonifier tablerotated. The MR fluid was checked periodically to ensure that themixture did not become too hot. A fan was used to cool the MR fluid.Once the cycle was complete, the jar was rotated to wash down anyparticles that were adhered to the walls of the jar. The sonificationstep was then repeated two more times. Once complete, the MR fluid wasremoved from the sonifier and a final stir of the MR Fluid was performedto ensure that there were no clumps in the MR fluid. The MR fluid wasthen placed in an oven for two hours at 50° C. (122° F.).

Example 2 Prosthetic Knee MR Fluid Loading

[0049] After the MR fluid was prepared, it was stirred to break up anysmall agglomerates. The MR fluid was then placed under the vacuumchamber stirrer and the bell jar was placed over the fluid and stirrer.The stirrer was turned on and vacuum was applied at 29.4″ Hg for 30minutes to extract residual air. The fluid was stirred until no bubblesappeared. The stirrer and vacuum were then turned off and the pressurewas slowly increased. The container was then removed from the vacuumchamber.

[0050] A measured volume of MR fluid was then transferred into a funnelinserted into the prosthetic knee actuator. The knee was placed in thevacuum chamber and the chamber was sealed with a bell jar. Vacuum wasslowly drawn to 28″ Hg. The knee was periodically agitated during thisprocedure.

[0051] The vacuum chamber was slowly filled with nitrogen gas to removethe vacuum. Vacuum was slowly released at about 2″ Hg per 10 seconds.The knee was agitated to help force the fluid into the knee. Nitrogenwas disconnected from the vacuum chamber when the gage read zero. Thevacuum chamber was then unsealed and the knee was removed. The funnelwas then removed from the knee. Care was taken so as to avoid tippingthe knee during this process, which would have resulted in a release ofthe nitrogen head. The knee was closed by inserting the appropriate setscrew with a torque of about 2.5 Nm applied to the screw.

Example 3 MR Fluid Settling Tests

[0052] Settling tests were conducted for thirteen different MR fluids.The rate of settling varied significantly and was found to be a functionof iron particle size, the use of additives, and the viscosity of thefluids.

[0053] Procedure for Settling Tests

[0054] MR fluids were formulated by adding carrier oil, with or withoutan additive, to a jar containing a weighed aliquot of carbonyl ironparticles. For formulations containing an additive, the additive wasblended with the carrier oil prior to mixing with the iron particles.The components were mixed by hand for several minutes and then the ironparticles were dispersed using high frequency ultrasonic energy suppliedby a Branson Digital Sonifier, Model 450. The fluids were subjected to2-3 cycles of ultrasonic energy, each cycle having a duration of 1.5minutes and power amplitude of approximately 50%. The fluids were thenmixed again by hand to insure complete dispersion of the iron particles.Fluids were not degassed prior to starting the settling tests.

[0055] Approximately 8 mL of each of the well-mixed MR fluids weretransferred to 10 mL graduated cylinders. The cylinders were closed byplacing a ground glass stopper into the neck of each cylinder.

[0056] The initial volume of MR fluid was recorded and the volume ofsettled material was read and recorded at regular intervals for a periodof twenty-one days. The fraction of settling was defined as the volumeof carrier oil, which separated from the MR fluid and floated on the topof the MR fluid divided by the initial fluid volume. TABLE 1 MR FluidsTested Dupont Particle Fluid 157 FSL Loading Component Additive BASF (%(v/v) of (% (v/v) of Carrier Fluid (% (v/v) of total Particle total MRtotal MR (% (v/v) of total fluid MR Fluid Type fluid) fluid) fluidcomponent) component) HQ81FS-28 HQ 28% 72%   95% Nye 8130 5% HQ85FSL-28HQ 28% 72%   95% Nye 8510 5% HS67FS5-32 HS 32% 68% 63.7% Nye 8510; 5%31.3% Dupont GPL-103 HS8510FSL-25 HS 25% 75%   95% Nye 8510 5%HS8510FSL-28H HS 28% 72%   95% Nye 8510 5% HS85FS10-28 HS 28% 72%   90%Nye 8510 10%  HS85FS10-32 HS 32% 68%   90% Nye 8510 10%  HS85FS1-32 HS32% 68%   99% Nye 8510 1% HS85FS5-32 HS 32% 68%   95% Nye 8510 5%OM8510-25 OM 25% 75%  100% Nye 8510 None OM85-25-1 OM 25% 75%   99% Nye8510 1% OMPF-25 OM 25% 75%  100% Nye 8130 None OMPFA-25 OM 25% 75%   95%Nye 8130 5%

[0057] Results

[0058] In general the largest iron particles, OM grade, settled thefastest, especially when the viscosity of the fluid was reduced byincluding the Dupont 157 FSL additive in the formulation. However, thesettling curves for the larger OM grade particles were initially steepand then leveled off after 10 to 14 days. Settling rates for the smalleriron particles, HS and HQ grade, were nearly linear over the twenty-oneday test period.

[0059] Overall, MR fluids with lower settling rates demonstrated longerlife and greater durability during subsequent bench testing inprosthetic knees. MR fluids with high settling rates produced hard cakedsettled iron particles. These fluids performed poorly in subsequentbench testing in prosthetic knees. MR fluids with low settling ratesproduce soft settled iron particles. These fluids generally performedwell in subsequent prosthetic knee bench tests.

Example 4 Viscosity and Shear Rate Testing

[0060] Dynamic viscosity of three mixed carrier oils and six MR fluidsmade from the mixed carrier oils were measured as a function of shearrate. Viscosity measurements were performed at ambient temperature (22°C.) using a Rheometric Scientific (TA Instruments) RFS-II rheometer witha parallel plate sample cell. All samples were run in duplicate withapproximately I cc of sample. Five of the samples were rerun induplicate on a second day due to incomplete mixing of the first samples.

[0061] Samples of mixed carrier fluids as well as MR fluids containingmixed carrier fluids were tested. The samples and viscosity measurementswere as follows: TABLE 2 Fluid Component BASF HS particles Viscosity(cP) Sample Name %((v/v)) %((v/v)) at 100 s⁻¹ A 100¹   0% 115 B 100²  0% 121 C 100³   0% 145 A-32 68¹ 32% 681 A-40 60¹ 40% 1371 B-32 68² 32%780 B-40 60² 40% 1651 C-32 68³ 32% 917 C-40 60³ 40% 1725

[0062] The dynamic viscosity curves which were measured at ambienttemperature (22° C.), for the samples above are illustrated in FIGS. 3,4, and 5. FIG. 3 represents the viscosity η, versus shear rate cures forthe three mixed carrier oils, A-C, and a sample of 100% ((v/v)) Nye 8510oil. All of the mixed carrier had a lower viscosity then Nye 8510. Thetypical viscosity specification value for Nye 8510 oil iss 65 cSt at 40°C., while the Dupont GPL-103 oil had a viscosity of 30 cSt at 40° C.

[0063]FIG. 4 represents the viscosity, η, versus shear rate curves forthe three mixed carrier oil MR fluids containing 32% ((v/v)) of HS ironparticles and a MR fluid which contains 100% Nye 8510 carrier oil and32% ((v/v)) of HS iron. Viscosities of the mixed carrier oil MR fluidswere less than the MR fluid containing only Nye 8510. This datademonstrated that it was possible to reduce the viscosity of a MR fluidby decreasing the viscosity of the carrier oil. Viscosity of the mixedcarrier oil MR fluids was lowered in proportion to the amount ofGPL-103, which was added to the carrier oil. The three mixed MR fluidsexhibited Non-Newtonian behavior as the viscosity of these fluidschanged continually with shear rate. The viscosity of these fluids wasapproximately six times that of the corresponding carrier oil at a shearrate of 100 s⁻¹.

[0064]FIG. 5 represents the viscosity, η, versus shear rate curves forthe three mixed carrier oil MR fluids which contain 40% ((v/v)) of HSiron particles. The viscosities of these fluids were considerably largerthan the viscosities of the MR fluids, which contained 32% ((v/v)) iron.Viscosity of the mixed carrier oil MR fluids containing 40% ((v/v)) ironwas lower for the fluids with higher amounts of GPL-103, however, theviscosity curve for B-40 was higher than expected. This apparent anomalywas mostly likely caused by incomplete mixing of the viscous 40% ((v/v))iron MR fluids prior to the viscosity measurements. The three MR fluidsexhibited Non-Newtonian behavior as the viscosity of these fluidschanged continually with shear rate. The viscosity of these fluids wasapproximately twelve times that of the corresponding carrier oil at ashear rate of 100 s⁻¹.

[0065]FIG. 6 summarizes the comparison of the viscosities of the threemixed carrier oils to Nye 8510 and the viscosities of the six mixedcarrier oil MR fluids to that of a MR fluid containing only Nye 8510.Nye 8510 had a viscosity of 240 cP at 100 s⁻¹, while the three mixedcarrier oils were well below 200 cP. The MR fluid which contained onlyNye 8510 and 32% ((v/v)) iron, namely HS8510FS5-32, has a viscosity of1,100 cP at 100 s³¹ ¹, while the MR fluids containing mixed carrier oilsand 32% ((v/v)) iron had viscosities of 680, 780 and 917 cP at 100 s⁻¹respectively, for the fluids which contain 50, 67 and 75% ((v/v)) of Nye8510. Viscosity of the MR fluids containing 32% ((v/v)) iron increasedwith increasing amounts of the more viscous Nye 8510. Viscosity of theMR fluids containing 40% ((v/v)) iron also increased with increasingamounts of Nye 8510.

[0066] Results indicated that the three carrier oils exhibited nearNewtonian behavior, while the six MR fluids all exhibited Non-Newtonianbehavior. Viscosity of the MR fluids were shown to be a function of bothiron loading and carrier oil viscosity. For MR fluids with 32% ((v/v))iron loading the viscosity was approximately six times that of thecorresponding carrier fluid. For MR fluids with 40% ((v/v)) iron loadingthe viscosity was about twelve times that of the corresponding carrierfluid. All of the MR fluids exhibited thinning i.e. a reduction inviscosity as a function of shear rate, especially at low shear rates.

Example 5 Prosthetic Knee Testing

[0067] Numerous prosthetics knees operating in shear mode were filledwith various MR fluids and tested for fluid performance, low-end torque,cavity pressure, and overall durability of the knee. Testing wasperformed to simulate use of the knee by an amputee. The knees weretested using a custom made test bench in conjunction with LabVIEW dataacquisition software (National Instruments). The test machine rotatedthe prosthetic knees at a rate of 32,000 cycles per day in order tosimulate accelerated knee usage. Prosthetic knees of varyingconfigurations were used with numerous MR fluid compositions. The goalwas to achieve three million cycles without knee failure. Due to limitedequipment, testing of several knees was cut short in order to test otherfluids and/or knee configurations. The following table illustrates someof the testing. TABLE 3 Fluid Name Fluid Composition* Fluid PerformanceDuration of Test OMPF-25 75% fluid component¹; Unit ran smoothly, offstate torque 2.2 million cycles 25% BASF OM at end of test was 0.7 N-m.particles. Applied field torque at end was 49 N-m. HS8510FSL-28 72%fluid component²; Unit ran well, off state torque at 1.2 million cycles28% BASF HS end of test was 0.8 N-m. Applied particles. field torque atend was 33 N-m. HQ8510FSL-28 72% fluid component²; Unit ran well, offstate torque at 2.4 million cycles 28% BASF HQ 2.2 million cycles was0.8 N-m. particles. Applied field torque at 2.2 million cycles was 43N-m. HS8510FSL-25 75% fluid component²; Unit ran well, off state torqueat 862,000 cycles 25% BASF HS 800,000 cycles was 0.6 N-m. particles.Applied field torque at 800,000 cycles was 40 N-m. HS67FSL-32 68% fluidcomponent³; Two units, A1 and A2, produce A1 - 433K cycles. 32% BASF HSimproved initial applied field A2 - 290K cycles. particles. torque 47-50N-m. Initial off state torque was in the range of 0.6-0.8 N-m. A1 at 433K was at 44 N-m. A2 at 290 K was at 47 N-m.

[0068] The various methods and techniques described above provide anumber of ways to carry out the invention. Of course, it is to beunderstood that not necessarily all objectives or advantages describedmay be achieved in accordance with any particular embodiment describedherein. Thus, for example, those skilled in the art will recognize thatthe composition may be made and the methods may be performed in a mannerthat achieves or optimizes one advantage or group of advantages astaught herein without necessarily achieving other objectives oradvantages as may be taught or suggested herein.

[0069] Furthermore, the skilled artisan will recognize theinterchangeability of various features from different embodiments.Similarly, the various features and steps discussed above, as well asother known equivalents for each such feature or step, can be mixed andmatched by one of ordinary skill in this art to perform methods inaccordance with principles described herein.

[0070] Although the invention has been disclosed in the context ofcertain embodiments and examples, it will be understood by those skilledin the art that the invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. Accordingly, the invention is notintended to be limited by the specific disclosures of preferredembodiments herein, but instead by reference to claims attached hereto.

What is claimed is:
 1. A magnetorheological fluid comprising ironparticles and a fluid component, wherein the fluid component comprises acarrier fluid and an additive; wherein the additive comprises aparafluoropropene and oxygen polymerized amide derivative and/or afunctionalized perfluorinated polyether fluid.
 2. The magnetorheologicalfluid of claim 1 wherein the iron particles range in size from about 0.2to about 50 microns.
 3. The magnetorheological fluid of claim 2 whereinthe iron particles range in size from about 0.4 to about 10 microns. 4.The magnetorheological fluid of claim 3 wherein the iron particles rangein size from about 0.5 to about 9 microns.
 5. The magnetorheologicalfluid of claim 1 wherein the iron particles comprise about 1 to about60% (v/v) of the total magnetorheological fluid volume.
 6. Themagnetorheological fluid of claim 5 wherein the iron particles compriseabout 10 to about 50% (v/v) of the total magnetorheological fluidvolume.
 7. The magnetorheological fluid of claim 6 wherein the ironparticles comprise more preferably from about 20 to about 40% (v/v) ofthe total magnetorheological fluid volume.
 8. The magnetorheologicalfluid of claim 1 wherein the carrier fluid is selected from the groupconsisting of silicone, hydrocarbon, esters, ethers, fluorinated esters,fluorinated ethers, mineral oil, unsaturated hydrocarbons, andcombinations thereof.
 9. The magnetorheological fluid of claim 8 whereinthe carrier fluid comprises one or more perfluorinated polyethers. 10.The magnetorheological fluid of claim 1 wherein the additive comprises aparafluoropropene and oxygen polymerized amide derivative.
 11. Themagnetorheological fluid of claim 1 wherein the additive comprises afunctionalized perfluorinated polyether fluid.
 12. Themagnetorheological fluid of claim 11 wherein the functionalizedperfluorinated polyether fluid additive comprises one or more functionalgroups selected from the group consisting of silane, phosphate,hydroxyl, carboxylic acid, amine dihydroxyl, ethoxy ether, isocyanate,aromatic, ester and alcohol functions.
 13. The magnetorheological fluidof claim 11 wherein the functionalized perfluorinated polyether fluidadditive comprises a poly(hexafluoropropylene epoxide) with a carboxylicacid located on the terminal fluoromethylene group.
 14. Themagnetorheological fluid of claim 1 wherein the additive comprises fromabout 0.1 to about 20% (v/v) of the fluid component.
 15. Themagnetorheological fluid of claim 14 wherein the additive comprises fromabout 1 to about 15% (v/v) of the fluid component.
 16. Themagnetorheological fluid of claim 15 wherein the additive comprises fromabout 2 to about 10% (v/v) of the fluid component.
 17. Themagnetorheological fluid of claim 1 comprising: about 28% (v/v) ironparticles; and about 72% (v/v) fluid component; wherein said fluidcomponent comprises about 5% (v/v) additive and about 95% (v/v)perfluorinated polyether carrier fluid.
 18. The magnetorheological fluidof claim 17 wherein the additive comprises poly(hexafluoropropyleneepoxide) with a carboxylic acid located on the terminal fluoromethylenegroup.
 19. The magnetorheological fluid of claim 17 wherein the additivecomprises a parafluoropropene and oxygen polymerized amide derivative.20. A magnetorheological fluid comprising iron particles and a fluidcomponent used in combination with a prosthetic knee; wherein the fluidcomponent comprises a carrier fluid and an additive; wherein theadditive comprises a parafluoropropene and oxygen polymerized amidederivative and/or a functionalized perfluorinated polyether fluid. 21.The magnetorheological fluid of claim 20 wherein the prosthetic kneeoperates in shear mode.
 22. The magnetorheological fluid of claim 20wherein the iron particles range in size from about 0.2 to about 50microns.
 23. The magnetorheological fluid of claim 21 wherein the ironparticles range in size from about 0.4 to about 10 microns.
 24. Themagnetorheological fluid of claim 22 wherein the iron particles range infrom about 0.5 to about 9 microns.
 25. The magnetorheological fluid ofclaim 20 wherein the iron particles comprise about 1 to about 60% (v/v)of the total magnetorheological fluid volume.
 26. The magnetorheologicalfluid of claim 25 wherein the iron particles comprise about 10 to about50% (v/v) of the total magnetorheological fluid volume.
 27. Themagnetorheological fluid of claim 26 wherein the iron particles comprisemore preferably from about 20 to about 40% (v/v) of the totalmagnetorheological fluid volume.
 28. The magnetorheological fluid ofclaim 20 wherein the carrier fluid is selected from the group consistingof silicone, hydrocarbon, esters, ethers, fluorinated esters,fluorinated ethers, mineral oil, unsaturated hydrocarbons, andcombinations thereof.
 29. The magnetorheological fluid of claim 28wherein the carrier fluid comprises one or more perfluorinatedpolyethers.
 30. The magnetorheological fluid of claim 20 wherein theadditive comprises a parafluoropropene and oxygen polymerized amidederivative.
 31. The magnetorheological fluid of claim 20 wherein theadditive comprises a functionalized perfluorinated polyether.
 32. Themagnetorheological fluid of claim 31 wherein the functionalizedperfluorinated polyether fluid additive comprises one or more functionalgroups selected from the group consisting of silane, phosphate,hydroxyl, carboxylic acid, amine dihydroxyl, ethoxy ether, isocyanate,aromatic, ester and alcohol functions.
 33. The magnetorheological fluidof claim 32 wherein the functionalized perfluorinated polyether fluidadditive comprises a poly(hexafluoropropylene epoxide) with a carboxylicacid located on the terminal fluoromethylene group.
 34. Themagnetorheological fluid of claim 20 wherein the additive comprises fromabout 0.1 to about 20% (v/v) of the fluid component.
 35. Themagnetorheological fluid of claim 34 wherein the additive comprises fromabout 1 to about 15% (v/v) of the fluid component.
 36. Themagnetorheological fluid of claim 35 wherein the additive comprises fromabout 2 to about 10% (v/v) of the fluid component.
 37. Themagnetorheological fluid of claim 20 comprising: about 28% (v/v) ironparticles; and about 72% (v/v) fluid component; wherein said fluidcomponent comprises about 5% (v/v) poly(hexafluoropropylene epoxide)with a carboxylic acid located on the terminal fluoromethylene groupadditive and about 95% (v/v) perfluorinated polyether carrier fluid. 38.The magnetorheological fluid of claim 20 comprising: about 28% (v/v)iron particles; and about 72% (v/v) fluid component; wherein said fluidcomponent comprises about 5% (v/v) parafluoropropene and oxygenpolymerized amide derivative additive and about 95% (v/v) perfluorinatedpolyether carrier fluid.
 39. The magnetorheological fluid of claim 20wherein the magnetorheological fluid is operable over a temperaturerange from about 10 to about 115° F.
 40. The magnetorheological fluid ofclaim 20 wherein the carrier fluid has a viscosity at 104° F. of about10 to about 100 cSt.
 41. The magnetorheological fluid of claim 40wherein the carrier fluid has a viscosity at 104° F. of about 30 toabout 80 cSt.
 42. The magnetorheological fluid of claim 41 wherein thecarrier fluid has a viscosity at 104° F. of about 50 to about 70 cSt.43. The magnetorheological fluid of claim 20 wherein the carrier fluidhas a viscosity index from about 100 to about 340 based on kinematicviscosity at 104 and 212° F.
 44. The magnetorheological fluid of claim43 wherein the carrier fluid has a viscosity index from about 120 toabout 320 based on kinematic viscosity at 104 and 212° F.
 45. Themagnetorheological fluid of claim 20 wherein the carrier fluid has apour point ranging from about −70° C. to about −40° C.
 46. Themagnetorheological fluid of claim 20 wherein the carrier fluid has apercent volatility at 121° C. ranging from about 0.01% to about 20%.